Raman spectroscopy is the name given to a technique whereby monochromatic electromagnetic radiation is used to irradiate a sample of material and the spectral distribution of the scattered radiation is analyzed. The incident radiation, which is traditionally in the visible region of the spectrum, excites the molecules of the material and causes them to undergo spontaneous emission of radiation. The frequency of the light that is scattered in this manner is modulated by the natural frequencies associated with the intramolecular and the intermolecular interactions of the molecules. The result is a series of spectral lines that are much weaker and shifted in frequency relative to the incident wavelength.
Because the intensity of the scattered radiation is weak compared to that of the incident radiation, the incident radiation must be of a relatively high intensity if the spectrum is to be observable. To provide a high intensity incident radiation, lasers typically are used as the light source for Raman spectroscopy.
A problem with Raman spectroscopy is the tendency of many sample materials to fluoresce when subjected to the incident radiation. This fluorescence dominates the signal and obscures the underlying Raman signal, which contains much of the useful information. As a result, until recently, Raman spectroscopy was mostly used in laboratory experiments with optically non-fluorescent compounds or with compounds with limited fluorescence.
If a near-infrared radiation source is used, as opposed to a visible light source, there is less fluorescence. However, disadvantages of near infrared spectroscopy are that the resulting signal is weak and infrared detectors are much less sensitive than those used for visible light.
The most recent applications of Raman spectroscopy for industrial samples use near-infrared excitation with laser radiation sources, infrared-sensitive detectors, and sophisticated Fourier transform data analysis. However, this methodology requires the availability of a sample of the material to be tested at the site of the test equipment. A sample must be collected off-line and transported and stored in a container suitable for the scattering process.
Attempts to achieve remote sample handling have accompanied advances in optic fiber technology. For example, absorption spectroscopy methods, as opposed to scattering methods, have attempted to achieve remote analysis of materials, using fiber optic transmission. These absorption methods have been only somewhat successful. Mid-infrared absorption methods attempt to avoid the fluorescence problems associated with visible and ultraviolet absorption methods, but suffer from problems with the optic fibers, such as high attenuation, fragility, and toxicity. Near-infrared absorption is suited to remote fiber optic sampling, but the response signal only contains a number of high frequency overtone bands, which are not useful for analysis of the fundamental vibrational frequencies of many organic and polymeric materials.
Optical fiber methods using scattering methods, such Raman spectroscopy, have also met with limited success. One recent patent, U.S. Pat. No. 4,802,761, discloses a method and apparatus that uses an optical fiber to transmit radiation from a laser to a sample-containing cell. Backscattered Raman radiation is then transmitted via collecting optical fibers to a detection system. The laser is a pulsed laser, tunable from 220 to 900 nanometers. The patent disclosure recognizes the problems associated with fluorescence, and the invention attempts to overcome these problems by tuning the laser to a different wavelength. Although the patent disclosure acknowledges that using longer incident wavelengths would alleviate fluorescence, it states that infrared spectroscopy is not amenable to fiber optic transmissions. Another patent, U.S. Pat. No. 3,906,241, also uses optical fibers for transmitting visible laser-generated light and collecting Raman radiation. The patent disclosure does not address the problems associated with fluorescence.
A recently published reference, Practical Raman Spectroscopy, edited by D. J. Gardiner and P. R. Graves (1989), notes the advantages of combining near infrared Raman spectroscopy with optical fiber technology, together with Fourier transform methods of analysis. This publication discusses the difficulty of removing the exciting line wavelength from the scattered radiation and an attendant need for Fourier transform instrumentation that is expensive and delicate.
Thus, a need exists for improved spectroscopic techniques that permit fiber optic transmission. These techniques should permit instrumentation that is rugged and inexpensive and thus suited for commercial and industrial use.